Band reject filters may be used in wireless network equipment, such as in base stations. Desirably, these band reject filters should be miniature in size to reduce the overall size of equipment in which they are installed. Surface acoustic wave, SAW, devices have been used to implement miniature band pass filters, but have not been widely used in band reject filter implementations, primarily due to a lack of demand for such implementations in wireless network equipment that are based on first generation, 1G, and second generation, 2G, wireless communication standards.
However, with the emergence of implementations of third generation, 3G, and fourth generation, 4G, wireless communication standards, frequency spectrum allocation is constrained, requiring very closely spaced frequency channels. This means that filters in a radio frequency, RF, front end must have steep transition bands to avoid interference. Preferably, steep transitions can be achieved with band reject filters having high Q. However, when high Q band reject filters are implemented using conventional components such as air cavity filters, such filters are undesirably large in size and are relatively expensive.
SAW resonators can be used to implement band pass filters and band reject filters. FIG. 1 shows a top view, and FIG. 2 shows a side view, of three known SAW resonators 10. As shown, the SAW resonator has inter-digital transducer, IDT, interlaced fingers 12 electrically connected by two IDT bus bars 14. In one embodiment 101, the SAW resonator has shorted reflector fingers 16 at each end that are shorted by reflector bus bars 18. In another embodiment 102, the SAW resonator has un-shorted reflector fingers at each end. In yet another embodiment 103, reflector fingers are altogether absent. The reflector fingers serve to enclose surface acoustic waves that emanate from the IDT fingers 12.
FIG. 3 is a top view of a known flip chip type SAW band pass filter configuration that includes 5 SAW resonators 10 electrically connected by connecting bus bars 20 in a ladder configuration. Solder balls 22 are provided to make electrical connections with bonding pads on a substrate 24 in order to provide input and output connections and connections to ground. The SAW resonators 10 and the connecting bus bars 20 are formed on a die 26 using conventional semiconductor forming processes, where the die may be cut from a crystal wafer, for example. The solder balls 22 may be formed on the die 26 or the substrate 24. In particular, in one example, the solder balls 22 can be formed on the die 26 after the SAW resonators 10 are formed on the die. The solder balls may be a good conducting metal such as tin, aluminum, copper, silver or gold, or a combination of conducting metals.
FIG. 4 is a side view of the band pass filter configuration of FIG. 3, showing that the SAW resonators 10 are formed on the side of the die 26 that faces the substrate 24. For a conventional band pass filter, the layout design of SAW electrodes shown in FIG. 3 is acceptable because the SAW resonators needed for the band pass filter design are very small. For example, for a 2 Gigahertz (GHz) SAW band pass filter design on a known substrate of 42 YX—LiTaO3, the dimensions of the largest SAW resonator are about 150 micrometers in length by about 100 micrometers in width. Therefore, it is possible to layout all of the SAW resonators that are needed for the band pass filter design to fit within an area of about or less than 2×2 millimeters. The dimensions of 2×2 millimeters are generally considered to be the maximum die size for using the flip chip type SAW assembly technique depicted in FIGS. 3 and 4 to manufacture the SAW filter. FIG. 5 is a block diagram of the SAW resonator configuration implemented in FIG. 3.
However, for a SAW band reject filter design, the SAW resonators need to have a high Q. Thus they are much bigger in dimension than the resonators needed for a band pass filter design. For example, for a 2 GHz SAW band reject filter design on a substrate of 42 STW Quartz, the layout dimensions of the largest individual SAW resonator of the filter design will be of dimensions about 1000 micrometers in length by about 300 micrometers in width. Therefore, the layout topology of FIG. 3 for the SAW band reject filter design would require a huge die and would further require long connecting bus bars that introduce undesirable excessive loss and parasitic inductance to the filter design.